1. Technical Field
The invention relates generally to propylene polymers, and in particular to articles prepared from propylene-ethylene copolymers.
2. Background of the Art
Propylene-based polymers are popularly used in a number of thermoplastic processing operations. These operations include, for example, injection molding to produce relatively thin-walled products. These polymers have conventionally been prepared via Ziegler-Natta catalysis, and the result has been found to be suitable for a wide variety of applications. However, some applications have indicated a need for improved properties, such as higher impact strength, lower melt temperatures, and higher clarity than has been conventionally attained in polypropylene. In order to obtain these properties those skilled in the art have learned that incorporation of a proportion of ethylene, to form propylene-ethylene random copolymers, offers some of these advantages, as well as, in some cases, greater flexibility and increased toughness.
Despite these improvements, however, there is still a need in the art for propylene-based polymers that exhibit, in particular, still greater improvements in clarity. This is because a number of the applications for which the propylene-ethylene random copolymers are particularly well-suited are also applications where transparency or a relatively high degree of translucency is desirable. These applications include, for example, bottles, which require high clarity as well as high toughness. One approach to obtaining such improvement in clarity has involved the use of so-called clarity-enhancing agents. These agents may be added to the molten polymer during pelletization, for example, or just prior to injection of the polymer into the mold. The clarity-enhancing agents generally operate by providing sites within the polymer for crystallization, which enables formation of smaller crystallites. Since crystallites result in diffraction or reflection of light, ensuring smaller crystallites reduces such occurrence, with the result that clarity is increased. Unfortunately, use of clarity-enhancing agents may increase the cost of preparing and using the polymers, and increasing their levels beyond a certain point may not be feasible or desirable because of potential detriment to other properties of the polymer.
Thus, what is desired in the art are articles prepared from propylene-ethylene random copolymers that exhibit improved clarity, with or without use of clarifying agents, and which also exhibit desirable levels of other physical properties such as toughness, flexibility, impact strength, and the like.